Unitary microwave transmit-receive duplex system with coaxial ring hybrid transformer

ABSTRACT

Unitary microwave transmit-receive apparatus for duplex operation comprises a pair of transmitters having different carrier frequencies, a common antenna, a pair of T-junction microwave switches, a common beating oscillator having a frequency substantially midway between the transmitting carrier frequencies, and a hybrid transformer composed of a coaxial ring conductor having two signal input coupling points, a beating oscillator coupling point and two output coupling points connected to a balanced modulator frequency changer. Each of the microwave switches is connected to a transmitter, the common antenna and one of the input coupling points of the coaxial transformer in a manner to enable a transmission on either carrier frequency and simultaneous reception on the other carrier frequency, and vice versa.

United States Patent [72] Inventor Alfred Kach Unterslggenthal, Switzerland 121] App]. No. 861,032

[22] Filed Sept. 25, 1969 [45] Patented Nov. 30, 1971 [73 Assignee Patelhold Patentverwertungs- 8: Elektro- Holding AG Glarus, Switzerland [32] Priority Oct. 15, 1968 l 3 3 Switzerland [54] UNITARY MICROWAVE TRANSMlT-RECEIVE DUPLEX SYSTEM WITH COAXIAL RING HYBRID TRANSMITTER Primary Examiner-Robert L. Griffin Assistant Examiner-James A. Brodsky Attorney-Greene & Durr ABSTRACT: Unitary microwave transmit-receive apparatus for duplex operation comprises a pair of transmitters having different carrier frequencies, a common antenna. a pair of T- junction microwave switches, a common beating oscillator having a frequency substantially midway between the transmitting carrier frequencies, and a hybrid transformer composed of a coaxial ring conductor having two signal input coupling points, a beating oscillator coupling point and two output coupling points connected to a balanced modulator frequency changer. Each of the microwave switches is connected to a transmitter, the common antenna and one of the input coupling points of the coaxial transformer in a manner to enable a transmission on either carrier frequency and simultaneous reception on the other carrier frequency. and vice FILTER FILTER PATENIEI] NDV30 I97! SHEET 1 [1F 3 JiNTENNA FILTE R TRANSMITTER ()SCILLATOR PUSH- PULL MIXER L/ ANTENNA FILTER TRANSMITTER INVENTgR ALFRED K CH BY ITHBL 4 77/ ATTORNEY PATENTEIJNUV30I971 3524508 swan 2 or 3 TlqA.

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l8 37 a i 5 39 ATTORNEY INVENTQB AL FRED K 46 H UNITARY MICROWAVE TRANSMIT-RECEIV E DUPLEX SYSTEM WITH COAXIAL RING HYBRID TRANSFORMER The present invention relates to a unitary microwave transmit-receive system for use in duplex operation of the type comprising a common transmitting and receiving antenna and a common beating oscillator and frequency converter or mixer in the receiver portion of said system for the generation of an intermediate frequency (IF).

In arrangements of this type operating according to the beating or heterodyning method, the same intermediate frequency may be produced by a receiving frequency being either above or below the oscillator or beating frequency, both variants resulting in the same frequency conversion loss. This fact is made use of in duplex operation, wherein the receiving frequency of one station is above and the receiving frequency of the counter station is below the oscillator or beating frequency. In arrangements for microwaves, it is furthermore necessary, in order to ensure a sufficient decoupling or selection by the antenna tuning filters, to utilize a relatively high IF (intermediate frequency). The receiving signal converted to such a relatively high IF, such for instance as 70 megacycles (mc.), is in turn converted, after adequate amplification, to a second lower IF, such for instance 10.7 me. The resultant signal is then subjected to final IF selection followed by further amplification and demodulation of the received signals.

In duplex operation, it is frequently required to utilize counter stations constructed in an identical manner or according to the unitary principle. In such a case, the circuits and parts must have an operating band width large enough to enable an interchange of the transmitting and receiving frequencies by a change of the oscillating frequency and readjustment of the frequency-determining or selective elements or devices of the system.

Moreover, in the case of mobile stations it is necessary to effect the change to a new operating frequency as expeditiously as possible. For this purpose, it would be necessary to provide frequency-selective elements at both counter stations capable of continuous tuning in synchronism with one another. Such synchronism is, however, difficult to achieve in practice within the range of higher microwave frequencies, in that multisection band-pass filters, such as four-section filters, may be tuned in rigid synchronism with each other over a relatively large frequency range up to only about 4 gigacycles (1 go. 1,000 mc.). Besides, modern microwave apparatus constructed substantially of semiconductor parts or devices includes frequency multiplier chains in both the transmitting and frequency converter sections which makes it impossible to realize a satisfactory and rigid synchronism on account of the large number of cooperating circuits or devices involved. Finally, the total band width of a multistage frequency multiplier is relatively small, i.e., of the order of 1.5-2 percent in the case of a multiplication ratio of l 480, and determined essentially by the characteristics of the capacitor diodes (varactors), so that it may not be increased materially.

Accordingly, among the objects of the present invention is the provision of a unitary microwave transmit-receive system or apparatus for use in duplex operation utilizing a pair of identical counter stations, which system will allow of an instant and expeditious change or interchange of the transmitting and receiving frequencies by relatively simple means, substantially without requiring any cumbersome and timeconsuming retuning of frequency-selective circuits or devices.

The invention, both as to the foregoing and anciliary objects as well as novel aspects thereof, will be better understood from the following detailed description, taken in conjunction with the accompanying drawings forming part of this disclo sure and in which:

FIG. 1 is a schematic diagram of unitary transmit-receive microwave apparatus constructed in accordance with the principles of the invention;

FIGS. 2 and 3 are partial diagrams of the hybrid ring transformer of FIG. 1, explanatory of the function of the invention;

FIG. 4 is a circuit diagram showing a balanced modulator suitable as a frequency converter for use in connection with the invention;

FIG. 5 is a part elevational and part sectional view, taken on line 5-5 of FIG. 6, illustrating a constructional embodiment of the hybrid ring transformer of FIG. 1;

FIG. 6 is a part plan and part sectional view taken on line 6-6 of FIG. 5;

FIG. 7 is a schematic diagram similar to FIG. 1, showing a modification of the system according to the invention; and

FIG. 8 shows diagrammatically further structural modifications of the coaxial hybrid ring transformer forming part of FIGS. 1 and 7.

Like reference numerals denote like parts throughout the different views of the drawings.

With the foregoing objects in view, the unitary microwave transmit-receive system according to the invention involves generally the provision of two transmitting paths, each com prising a transmitter, a T-junction microwave switch and an antenna tuning filter and being connected to a common antenna, a hybrid microwave transformer in the form of a coaxial ring conductor having three input coupling points one of which is connected to a common beating (heterodyning) oscillator and the remaining two coupling points being alternatively connectable to the microwave switch of the coordinated transmitting path. The electrical length between any two coupling points of said conductor is equal to 1\/2 or a whole number multiple thereof, wherein A represents the operating frequency. The ring conductor is furthermore fitted with a pair of output coupling points disposed each on one side of the oscillator input coupling point and spaced from the latter by an electrical distance equal to )t/4 or an odd multiple thereof, both said coupling points being connected to the input of a balanced modulator serving as a frequency converter. As a consequence, there is enabled by the use of a system of this type a transmission upon either transmitting frequency and simultaneous reception on the frequency corresponding to the other transmitting frequency, and vice ver- By the selection of a proper relationship between the heating or oscillating frequency to the transmitting frequencies, that is with the former being about midway between the latter frequencies, and uni-control of the transmitters and connection of the microwave switches with the proper input coupling points of the hybrid transformer, an interchange of the transmitting and receiving frequencies of a pair of unitary counter stations may be efiected practically instantly and in relatively simple manner for the carrying out of a duplex operation, in a manner as will become further apparent as the description proceeds in reference to the drawings.

In the foregoing, the wave length A applies to both the receiving and beating or oscillating frequencies which, in the case of microwaves within the range concerned, result in electrical spacings as described hereinbefore so close to one another as to become practically negligible.

FIG. 1 shows a first variant of the transmit-receive system of the invention, comprising two transmitting paths each of which includes respectively a transmitter l, 2, a cavity resonator 3, 4, a microwave switch 5, 6 and a selective antenna filter 7, 8 in series and is connected to the common antenna 9. Each of the microwave switches consists, in the example shown, of four star'connected coaxial sections of which sections 11 and 10 are connected respectively to the associated antenna filters 7, 8 and to the transmitters I, 2 via the cavities 3, 4, while the further section 12 forms a compensated M4 or quarter wave stub support and the last section 13 is fitted with a displaceable central contact 14. The balanced modulator frequency converter comprises essentially a transformer in the form of a hybrid coaxial ring conductor 15 having three input coupling points 16, 17 and 18 arranged with the electrical lengths between two adjacent coupling points being equal to M2. Connectible to each of the coupling points 16 and 17 is one of the switching contacts 14 of the associated microwave switches 5,

6, while the third coupling point 18 is connected to the output of a local (beating) oscillator 19. Further provided on either side of the coupling point 18, at electrical distances of M4, are a pair of output coupling points 20, 21 connected each to one of the mixing diodes 22, 23 of a balanced modulator 23 more clearly described hereafter in reference to FIG. 4.

Sections 12 of the microwave switches 5, 6 acting as a M4 or quarter wave stubs and sections 13 fitted with the contacts 14 have a common inner conductor in the fon'n of a sleeve 24 in which is disposed an axially displaceable switching rod 25 to the end of which is secured the contact 14. In order to interrupt the transmitting paths, the cavities 3, 4 are fitted, in the example shown, with tuning pistons 26 which, upon entering the cavities, result in a detuning such as to block or interrupt the energy transmission. Pistons 26 together with the switching rods 25 of the microwave switches are mechanically connected through a common linkage 27 or the like uni-control operating member for effecting an interchange of the transmitting and receiving frequencies, in a manner as will become more apparent from the description of the function of the invention as follows.

The electrical length of the sections 10 and 13 of the microwave switches 5, 6 is so chosen that, with the respective transmitting path being interrupted, their reactance at the star points of the switches is either infinity or at least of a high ohmic value. As is understood, the transmission paths should be as free from deflections as possible. By the use of relatively simple means analogous to the H4 supporting stubs, broad band compensation is possible in practice.

The operation of the system according to FIG. 1 is as follows.

In the position of the operating member 27 shown in FIG. 1, the upper transmission path is connected to the antenna 9 via the cavity 3, the microwave switch 5 and filter 7, while the connection between the switch 5 and the input coupling point 16 of the conductor is interrupted. At the same time, the lower transmitting path is interrupted by the piston 26 traversing the cavity 4, whereby receiving signals are enabled to pass from the antenna 9 to the frequency converter via the filter 8, the closed contact 14 of the microwave switch 6 and the coupling point 17.

The frequency conversion or mixing of an input signal with the local oscillating (beating) signal will now be explained in reference to FIGS. 2 and 3.

The ring conductor 15 acts as a hybrid transformer of the type shown and described by Swiss Pat. No. 271,529. FIG. 2 again shows the conductor in the position according to FIG. 1. The receiving or input signal E, and the local oscillating signal 0 are split at the respective input coupling points 17 and 18 into two components travelling in opposite directions along the conductor 15. Inasmuch as the difference of the paths travelled by the components of the input signal E, from its coupling point 17 to each of the output coupling points 20, 21, and inasmuch as furthermore the difference of the paths travelled by the components of the oscillating signal 0 from its coupling point 18 to the output coupling points 20, 21, are equal to A or zero, these signal components are combined in proper phase relation at the coupling points 20, 21, that is, with the components originating from the input signal E, being in phase opposition and the components originating from the oscillating signal 0 being in phase with one another. The composite signals at the output coupling points are applied to the respective mixing diodes 22 and 23 of the balanced modulator 23'.

As pointed out herein above, the difference between the wave length of the input signal E, and the oscillating signal 0, for the frequencies concerned in the microwave range, is practically negligible, whereby to result in substantially the same relationship of the electrical distances between the various coupling points for both the received and local oscillating signals, respectively.

Inasmuch as the coupling points 17, 18 of the ring conductor 15 are spaced from each other by electrical distances equal to M2, the components of the signal applied to one of these points and travelling in opposite directions along the conductor, cancel one another at the other coupling point.

FIG. 3 shows the conditions for the other operating position of the member 27. In this position, the piston 26 has been withdrawn from the cavity 4 and the contact 14 of the microwave switch 6 has been disengaged from the coupling point 17, while the cooperating piston 26 has entered the cavity 3 and the contact 14 of the microswitch 5 hasbeen brought into contact with the coupling point 16 of the conductor 15, FIG. 3. As a consequence the lower transmitting path is now operably connected with the antenna 9 via the cavity 4, microwave switch 6 and filter 8, while the upper transmitting path is interrupted. The input signal E FIG. 3, is applied, via the antenna 9, filter 7, and microwave switch 5, to the input coupling point 16 of the conductor 15 and superimposed upon the oscillator signal 0 at the output coupling points 20, 21 connected to the balanced modulator, in substantially the same manner as the signal E,, FIG. 2.

In both operating positions of the device, one of the transmitters is disconnected by the detuning of the cavity 3, 4 in its respective transmitting path. The cavities 3, 4 may constitute the output circuits of a frequency multiplier or of a parametric up-conversion or down-conversion mixer. In the case of a frequency multiplier utilizing a capacitive diode, a detuning occurs due to the reactance change of the diode upon disconnection of the transmitter, which may make it possible to dispense with an additional detuning by means of the pistons 26.

The balanced modulator may be of conventional construction, such as shown by FIG. 4. According to the latter, the composite signals, appearing at the output coupling points 20, 21 and originating from the input signals E, or E and the local oscillating signals 0, are applied respectively, via the mixing diodes 22, 23, to filter capacitors 28, 29, while RLC-circuits 30, 31 serve for the deviation and adjustment of the diode biasing currents. The intermediate frequency signal is derived, via further capacitors 32, 33 and applied, via the output 34, to the IF input filter of the receiver.

A preferred constructional embodiment of the coaxial ring conductor 15 and mixing diodes 22, 23 is shown in FIGS. 5 and 6. According to the latter, the ring conductor consists of a cup-shaped casing 35 closed by a cover 36, to act as the outer conductor of the coaxial line 15, FIG. 1. Connected to the casing 35 are the coaxial line sections 13 of the microwave switches 5, 6 and a third coaxial section 37 for the feeding of the oscillating signal. The ring conductor is constituted by an inner striplike conductor 38 connected, at the output coupling point 18, with the inner conductor 39 of the oscillator feed section 37 and fitted, at its coupling points 16 and 17, with suitable knife switches or connectors. One of the latter engages the associated switching contact 14 connected to the associated microswitch via the respective switching rod 25.

In order to support the strip conductor 38, the coupling points 20, 21 are fitted with )\/4 stubs 40, 41 terminating in connecting sleeves 42, 43 which project into tubes 44, 45 extending from the cover 36. The tubes 44, 45 are closed by caps 46, 47 within which are insulatingly mounted the diodes 22, 23, in such a manner as to electrically connect the same with the coupling points 20, 21 of the conductor 38 of the ring line 15 via the diode contact terminals and sleeves 42, 43. The remaining terminals of the diodes 22, 23 are connected, within the caps 46, 47, to the associated coaxial conductors 48, 49 leading to the balanced modulator, FIG. 4.

In order to match the impedance of the ring line 15 with its connecting lines, the wave impedance of the line, as can be easily seen, should be greater by a factor oft 2 than the wave impedance of the connecting lines. The impedance characteristics of the reactive portions in both the transmitting and receiving sections of the device, such as the M4 stubs, should be compensated throughout and over a broad frequency band.

According to a preferred example of a transmit-receive system according to the invention utilizing frequency multiplication for the generation of both the transmitting and heterodyning frequencies; both antenna filters have a bandpass width of 80 mc. located above and below a main operating frequency of about 8,000 mc. Within this frequency range of 80 me. it is possible to accommodate for instance 50 transmitting channels having a 1 me. spacing between the mean channel frequencies and selectable by changing the fundamental frequency by the switching of crystal filters, while all the remaining frequency-determining elements of the system remain unchanged.

A further variant of the transmit-receive system according to the invention utilizing electronic switching is shown by FIG. 7. According to the latter, the transmitter l, 2 of each transmitting path is connected via two ports of a three-port circula tor 50, 51 with the respective antenna filter 7, 8, while the input coupling points 16, 17 of the line are connected each with one of the third ports of the circulator via electronic diode switching chains 52, 53. The latter each consist, in the example shown, of three diodes arranged at electrical distances of M4 along I the connecting line to the input coupling points 16, 17 said diodes being connectable to a biasing voltage source via the parallel-connected remaining diode terminals 54, 55. The transmitting signals pass through the input and output ports of the circulators 50, 51 in the direction of the arrows, whereby the third ports of the circulators receive only the reflected portion of the energy not radiated by the antenna 9. in order to interrupt the connections between the circulators 50, 51 and the associated input coupling points 16, 17, the diodes 52, 53 of the respective chain are biased by a voltage in the current-passing direction, whereby to increase the damping of the microwave signals to an extent as to prevent any disturbing signals to reach the coupling points 16, 17 of the ring line 15. In place of diodes as switching devices, ferrite or the like, electronic switches may be used for the purpose of the invention.

While the hybrid ring conductors according to the modifications shown in the foregoing are of circular shape, it is understood that their shape may deviate therefrom, it being merely required that the line has a constant wave impedance and that the electrical distances are the same as described in connection with the circular line. FIG. 8 shows for illustration purposes lines of different shape with FIG. 8a relating to the circular line, FIG. 8b showing a line'with two-thirds of its length having a curved configuration, and FIG. 8c showing a line of rectangular configuration.

In the arrangement shown, .the pistons 26 serve to disconnect either transmitter 1, 2 during transmission by the other transmitter, and vice versa, the pistons or equivalent means being advantageously synchronized with the switches 5, 6 by the aid of the uni-control operating member 27. As will be understood, other suitable means may be provided for the disconnection of the transmitters, such for instance as voicecontrolled switches as commonly used in duplex operation. Thus, in FIG. 7 a voice or other modulating signal applied to the transmitter 1 may serve to disconnect the transmitter 2, and vice versa.

1n the foregoing the invention has been described in reference to an illustrative or exemplary device. It will be evident, however, that variations and modifications as well as the substitution of equivalent parts and devices for those shown for illustration, may be made without departing from the broader scope and spirit of the invention.

lclaim:

. 1. Unitary microwave transmitting and receiving apparatus comprising in combination:

1. a transmit-receive antenna,

2. a first and a second signal path each connected to said antenna and including, in the order named, an antenna band-pass filter, a T-junction, microwave switch, and a transmitter,

3. said transmitters having different carrier frequencies and said filters having band-pass characteristics to encompass the signal frequency band of the respective transmitters,

4. a heterodyning oscillator having a frequency substantially midway between said carrier frequencies, A,

5. a balanced modulator frequency changer,

6. a hybrid microwave conductor closed upon itself and having a. first, second and third input coupling points equally spaced along said conductor by electrical distances equal to )t/2 or a whole number multiple thereof.

b. means connecting said first input coupling point to said oscillator and for connecting said second and third input coupling points alternatively to one of said filters via the associated microswitches, and

c. a pair of output coupling points upon said conductor located on either side of and spaced from said first input coupling point by electrical distances equal to M4 or an odd multiple thereof, each of said output coupling points connected to a different signal input of said balanced modulator frequency changer, and

7. control means in each of said signal paths to disable either of said transmitters during transmission by the other transmitter, and vice versa,

8. whereby to enable a signal transmission on either carrier frequency by one of said transmitters and simultaneous signal reception on the other carrier frequency via said frequency changer, and vice versa.

2. Microwave transmitting and receiving apparatus as claimed in claim 1, wherein each of said microwave switches consists of four star-connected line sections two of which are connected to the respective transmitter and antenna filter, a

third of which sections constitutes a quarter wave supporting stub, and the fourth section includes an inner axially displaceable shifting rod carrying a contact arranged for operation into engagement with one of said second and third input coupling points of said microwave conductor.

3. Microwave transmitting and receiving apparatus as claimed in claim 2, wherein the line sections of said switches and said microwave conductor are in the form of coaxial lines.

4. Microwave transmitting and receiving apparatus as claimed in claim 1, wherein each of said microwave switches consist of four star-connected coaxial line sections two of which are connected to the respective transmitter and antenna filter, a third of which sections includes an inner axially displaceable shifting rod carrying a contact arranged for operation into engagement with one of the second and third input coupling points of said conductor, and the fourth of which sections has an inner conductor acting as a quarter wave supporting stub for the respective microwave switch and forming a sleeve supporting the shifting rod of said switch.

5. Microwave transmitting and receiving apparatus as claimed in claim 4, including uni-control operating means for said switching rods, to connect one of the rods with its coordinated coupling point while disconnecting the other rod from its coordinated coupling point of said conductor, and vice ver- 6. Microwave transmitting and receiving apparatus as claimed in claim 1, including a resonant cavity interposed between each of said transmitters and the associated microswitch, and wherein said control means is comprised of a tuning piston for each of said cavities synchronized with the associated microswitch.

7. Microwave transmitting and receiving apparatus as claimed in claim 1, including uni-control means to connect the transmitter in said first path and to disconnect the transmitter in said second path by said control means and to simultaneously disconnect said conductor from said first path and to connect it to said second path, and vice versa.

8. Microwave transmitting and receiving apparatus as claimed in claim 1, wherein said microwave switches consist of three-port circulators having two ports connected to the associated transmitter and antenna filter and being excited magnetically to enable energy transmission from the transmitters to said antenna, and a pair of electronic switches each connected between the third port of one of said circulators and the associated input coupling point of said conductor.

9. Microwave transmitting and receiving apparatus as claimed in claim 8, wherein each of said electronic switches consists of a chain of parallel-connected diodes spaced by electrical distances of M4, and means to temporarily energize said diodes in the current-passing direction. 

1. Unitary microwave transmitting and receiving apparatus comprising in combination:
 1. a transmit-receive antenna,
 2. a first and a second signal path each connected to said antenna and including, in the order named, an antenna band-pass filter, a T-junction, microwave switch, and a transmitter,
 3. said transmitters having different carrier frequencies and said filters having band-pass characteristics to encompass the signal frequency band of the respective transmitters,
 4. a heterodyning oscillator having a frequency substantially midway between said carrier frequencies, lambda ,
 5. a balanced modulator frequency changer,
 6. a hybrid microwave conductor closed upon itself and having a. first, second and third input coupling points equally spaced along said conductor by electrical distances equal to lambda /2 or a whole number multiple thereof. b. means connecting said first input coupling point to said oscillator and for connecting said second and third input coupling points alternatively to one of said filters via the associated microswitches, and c. a pair of output coupling points upon said conductor located on either side of and spaced from said first input coupling point by electrical distances equal to lambda /4 or an odd multiple thereof, each of said output coupling points connected to a different signal input of said balanced modulator frequency changer, and
 7. control means in each of said signal paths to disable either of said transmitters during transmission by the other transmitter, and vice versa,
 8. whereby to enable a signal transmission on either carrier frequency by one of said transmitters and simultaneous signal reception on the other carrier frequency via said frequency changer, and vice veRsa.
 2. a first and a second signal path each connected to said antenna and including, in the order named, an antenna band-pass filter, a T-junction, microwave switch, and a transmitter,
 2. Microwave transmitting and receiving apparatus as claimed in claim 1, wherein each of said microwave switches consists of four star-connected line sections two of which are connected to the respective transmitter and antenna filter, a third of which sections constitutes a quarter wave supporting stub, and the fourth section includes an inner axially displaceable shifting rod carrying a contact arranged for operation into engagement with one of said second and third input coupling points of said microwave conductor.
 3. Microwave transmitting and receiving apparatus as claimed in claim 2, wherein the line sections of said switches and said microwave conductor are in the form of coaxial lines.
 3. said transmitters having different carrier frequencies and said filters having band-pass characteristics to encompass the signal frequency band of the respective transmitters,
 4. a heterodyning oscillator having a frequency substantially midway between said carrier frequencies, lambda ,
 4. Microwave transmitting and receiving apparatus as claimed in claim 1, wherein each of said microwave switches consist of four star-connected coaxial line sections two of which are connected to the respective transmitter and antenna filter, a third of which sections includes an inner axially displaceable shifting rod carrying a contact arranged for operation into engagement with one of the second and third input coupling points of said conductor, and the fourth of which sections has an inner conductor acting as a quarter wave supporting stub for the respective microwave switch and forming a sleeve supporting the shifting rod of said switch.
 5. Microwave transmitting and receiving apparatus as claimed in claim 4, including uni-control operating means for said switching rods, to connect one of the rods with its coordinated coupling point while disconnecting the other rod from its coordinated coupling point of said conductor, and vice versa.
 5. a balanced modulator frequency changer,
 6. a hybrid microwave conductor closed upon itself and having a. first, second and third input coupling points equally spaced along said conductor by electrical distances equal to lambda /2 or a whole number multiple thereof. b. means connecting said first input coupling point to said oscillator and for connecting said second and third input coupling points alternatively to one of said filters via the associated microswitches, and c. a pair of output coupling points upon said conductor located on either side of and spaced from said first input coupling point by electrical distances equal to lambda /4 or an odd multiple thereof, each of said output coupling points connected to a different signal input of said balanced modulator frequency changer, and
 6. Microwave transmitting and receiving apparatus as claimed in claim 1, including a resonant cavity interposed between each of said transmitters and the associated microswitch, and wherein said control means is comprised of a tuning piston for each of said cavities synchronized with the associated microswitch.
 7. Microwave transmitting and receiving apparatus as claimed in claim 1, including uni-control means to connect the transmitter in said first path and to disconnect the transmitter in said second path by said control means and to simultaneously disconnect said conductor from said first path and to connect it to said second path, and vice versa.
 7. control means in each of said signal paths to disable either of said transmitters during transmission by the other transmitter, and vice versa,
 8. whereby to enable a signal transmission on either carrier frequency by one of said transmitters and simultaneous signal reception on the other carrier frequency via said frequency changer, and vice veRsa.
 8. Microwave transmitting and receiving apparatus as claimed in claim 1, wherein said microwave switches consist of three-port circulators having two ports connected to the associated transmitter and antenna filter and being excited magnetically to enable energy transmission from the transmitters to said antenna, and a pair of electronic switches each connected between the third port of one of said circulators and the associated input coupling point of said conductor.
 9. Microwave transmitting and receiving apparatus as claimed in claim 8, wherein each of said electronic switches consists of a chain of parallel-connected diodes spaced by electrical distances of lambda /4, and means to temporarily energize said diodes in the current-passing direction. 